Flights heading east are usually faster than the same flights heading west because of the jet stream: a band of strong wind, high in the atmosphere, that blows from west to east across the mid-latitudes. Eastbound, planes ride it as a tailwind; westbound, they either fight it as a headwind or detour around it. Across the Atlantic the difference is often an hour or more.
What the jet stream is
Jet streams are rivers of fast-moving air near the top of the troposphere, around 30,000 to 40,000 feet, right where airliners cruise. They form along the boundaries between cold polar air and warmer air further south, and the Earth's rotation turns them eastward. The polar jet stream, the one that matters most for flights across the North Atlantic and North Pacific, commonly blows at 100 knots or more, and sometimes above 200. It's strongest in winter, when the temperature contrast is biggest.
How much it matters
Take New York to London, 5,540 km on the great circle. At a typical cruise of 480 knots with no wind, that's about 6h 14m in the air. With a 60-knot average tailwind, it's 5h 32m; against a 60-knot headwind, 7h 7m. That's a difference of 1h 35m from wind alone, and on days with a strong jet, eastbound flights can land well ahead of schedule.
Simplified, with a steady wind along the whole route. Real flights see the wind change along the way, and the figures leave out taxiing, climb and descent.
Why planes don't just fly the shortest route
Because with a 150-knot wind in play, the shortest path isn't the quickest. Every day, air traffic controllers in the UK and Canada publish a fresh set of North Atlantic Tracks, routes placed to catch the jet stream eastbound and to avoid its core westbound. That's why your westbound flight might swing far north or south of the eastbound one. For more on why routes curve at all, see why flight paths look curved.
Is it the Earth's spin?
A common guess is that planes fly faster east because the Earth rotates eastward underneath them. They don't: the atmosphere rotates with the Earth, so a plane flying through it doesn't gain or lose anything from the spin directly. The rotation's real role is indirect, steering the winds that form the jet stream.
Speed records
On nights with an exceptionally strong jet stream, transatlantic flights have recorded ground speeds above 800 miles per hour, faster than the speed of sound at sea level, while their speed through the air stayed perfectly normal. Ground speed is airspeed plus wind; the knots converter handles the units.
Plan your trip
The flight time calculator estimates how long any route takes, the same in both directions, since winds change daily. Airline timetables bake in the typical wind, which is why the westbound schedule is longer. In Flight, your focus session is a flight at one steady speed, so New York to London lasts exactly as long either way.